Enhancing Aluminum Reactivity by Exploiting Surface Chemistry and Mechanical Properties

Metal-fluoropolymer based systems have drawn attention in the combustion community in recent years due to the exothermic surface chemistry between fluorine F and the alumina Al2O3 shell surrounding aluminum Al fuel. Incorporating a liquid fluorinated oligomer, specifically perfluoropolyether PFPE, exhibits this surface chemistry while increasing the proximity of fuel and oxidizer. The results show that the combustion performance of these blends is highly dependent on the Al2O3 concentration and exposed surface area. There is a balance to optimizing Al particle reactivity with PFPE coating between activating Al particles with exothermic surface chemistry versus the unreacted alumina that contributes a thermal heat sink during energy generation. To further understand the role of PFPE in energetic systems, varying concentrations were blended with AlCuO and AlMoO3 thermites. Results show that the performance of the thermite-PFPE blends is highly dependent on the bond dissociation energy of the metal oxide. Fluorine-aluminum based surface exothermic chemistry with MoO3 produce an increase in reactivity while the blends with CuO show a decline when increasing the PFPE loadings. These results provide new evidence that optimizing aluminum combustion can be achieved through activating exothermic Al surface chemistry that produces aluminum fluoride. Another avenue for increasing Al reactivity is to alter its mechanical properties. In bulk material processing, annealing and quenching metals such as Al can relieve residual stress and improve mechanical properties. Aluminum particles underwent thermal treatment in order to examine the effect of annealing and quenching on the strain of Al particles and the corresponding reactivity of Al and CuO composites. Flame propagation experiments also show thermal treatments effect reactivity when combined with CuO. These results show that altering the mechanical properties of Al particles affects their reactivity.